Inherent limitations to gas turbine power section rotating machinery vis-à-vis thermal stress inhibiting higher combustion temperatures and thus limiting fuel efficiency force designers to adopt complex mechanical interventions to achieve higher thrust potential, e.g., high-bypass turbofan disks, while leaving nearly half of the thermodynamic energy potential available from typical aviation fuels unexploited because burning them at their highest stoichiometric combustion temperatures would cause highly-stressed power turbine rotating machinery to fail. Inordinately high fuel consumption is also expended while aircraft are ground taxiing on main engine power, idling, or left at low-output settings such as when throttled back on descent-to-landing phases of aircraft operations. A solution to the thermal stress problem in gas turbine engines is still elusive and pertinent, given the cost of aviation jet fuel that is now impacting commercial airline financial solvency and air travel affordability, such that the adversely high low-power setting fuel consumption of said engines also needs to be solved.
In prior art devised to offset the limitation of thermal stresses subjected to hot section power turbine rotating machinery in turbojet/turbofan engines, the inventions of U.S. Pat. No. 3,678,306, to Gamier et al.; U.S. Pat. No. 4,051,671, to Brewer; and U.S. Pat. No. 4,368,620, to Giles, seem most relevant. In all said prior art, they did not address excessively high fuel consumption when powering a turbocompressor over extended periods at high altitudes using the devices they taught, given limitations to those devices taught while used in the stratosphere, or flexibility over a wide range of altitudes, or any means to ameliorate excessive fuel consumption when gas turbines as they taught are throttled back during descent and landing phases of a flight, or on the ground during taxiing maneuvers, when queuing for an active runway or idling after engine start.
Ram air turbines (RAT) as mentioned by Brewer and Giles are conventionally simple, small, retractable devices that lack the flexibility to contribute significantly to the creation of propulsion throughout a flight mission at varying altitudes, speeds and configurations. Further, Giles taught a windmill for ramjet engines, wherein the operative windstream is induced by ramjet engine jet exhaust within a closed system. Said art does not entertain powering the compressor work of a large main driver engine as used by commercial transport aircraft. No design feature was offered to improve fuel efficiency for said engines at various speeds and altitudes.
Conventional aviation turboshaft-powered auxiliary power units (APU) as mentioned in prior art, being generator sets as typically installed on jet aircraft of today, do not provide power levels sufficient to electrically drive the turbocompressors of the main turbojet/turbofan driver engines on any given size category of jet aircraft on which they are installed. If said APUs were logically scaled upward through the use of higher output turboshaft engines used by the APU to primarily support the powering of main engine turbocompressors, that would only mean that they would be replacing internal hot section power turbines in a conventional turbojet/turbofan main driver engine with a separate turboshaft engine that could might be more fuel efficient overall at low altitudes if the main driver jet engine—by freedom from hot turbines in said engine achieves lower thrust specific fuel consumption (TSFC) via combustion of air/fuel mixtures at higher flame temperatures—but would be a much less efficient proposition at higher altitudes due to contemporary turboshaft engine torque output limitations at stratospheric altitudes, as well as introducing another heat engine system component with its own intrinsic mechanical losses, and also suffering from the same thermal stress limitations that Brewer and Garnier proposed to overcome in main driver engines.